When a work machine such as a telehandler is carrying a payload over rough terrain, the hydraulic boom holding the payload experiences shocks from movements of the payload. These shocks are usually transferred directly to the machine via the boom. This makes the machine more susceptible to pitch and bounce, resulting in an uncompromising ride and an increase in operator fatigue. Hydraulic ride control circuits, that is hydraulic circuits that improve the ride quality of a work machine, are known. Such circuits conventionally selectively connect a hydraulic accumulator with the hydraulic ram arrangement of the boom in order to cushion any shocks experienced by the boom and ram. In cushioning the shocks, the circuit will normally permit a limited inward or outward movement of the ram (e.g. ±50 mm).
One example of such a circuit is disclosed in GB 2365407A to JC Bamford Excavators Limited. In GB '407, the hydraulic boom circuit includes a main control valve connected via first and second fluid lines to first and second sides of the hydraulic ram, respectively. By allowing pressurised fluid to flow into one side of the ram while simultaneously draining fluid from the other side of the ram back to a hydraulic reservoir, the control valve controls the movement of the ram and, consequently, the raising and lowering of the boom. For safety reasons, a hose burst valve, otherwise known as a load hold valve, is provided in the fluid circuit such that the ram will remain held in position should a flexible hose burst in the circuit between the control valve and the load hold valve.
In order to provide the cushioning effect, GB '407 includes an accumulator between the load check valve and the first side of the ram. A secondary control valve allows the accumulator to accumulate charge pressure during normal operation of the boom. When the ride control circuit of GB '407 is activated, the secondary control valve is energized and permits two-way flow between the accumulator and first side of the ram, the accumulator thus cushioning, via the ram, the shocks experienced by the boom during operation.
Furthermore, GB '407 also discloses the use of a further secondary control valve that controls fluid flow from the second side of the ram to a low pressure fluid reservoir. As with the other secondary control valve, this valve is opened when the ride control circuit is activated, thereby allowing fluid to drain from the second side of the ram to the reservoir should the ram move outwards by any degree when the ride control circuit is in operation.
One disadvantage with the system disclosed in GB '407 is that with the accumulator located between the load check valve and the first side of the ram, there is no safety mechanism to prevent the dropping of the boom should there be a sudden pressure loss in the accumulator, which could be caused by a burst hose, for example. Furthermore, as fluid from the second side of the ram is free to drain to a low pressure reservoir when the ride control circuit is engaged, the ram (and boom) are only effectively cushioned on one side, i.e. the first side of the ram, as no pressurised fluid remains on the second side of the ram.
It is an aim of the present invention to obviate or mitigate one or both of the aforementioned disadvantages.